HDPE was functionalized with two different esters in an inert atmosphere at 160°C, under different experimental conditions by thermolysis method. The order of the functionalization and crosslinking reactions was determined. The C=C formation peak is assigned at 1610 cm-1.The percentage of functionalization was determined by FTIR spectroscopy method. Thermogravimetric analysis and differential scanning calorimetric methods were also used to study about the thermal properties of the functionalized HDPE.